Unknown is the effect of hypothermia or various cardioplegic solution compositions at various temperatures on interstitial myocardial pH during global ischemia and reperfusion. Hydrogen ion flux is very rapid in ischemic tissue and expecially so during the initial 30-90 seconds of reperfusion. An NIH developed fiberoptic system using membrane captured phenolphthalin can measure pH in tissues continuously although the time constant is long at present. These studies were designed to test the hypothesis that the exponential rate of rise of hydrogen ion concentration in ischemic myocardium is altered substantially by hypothermia because of known pKa changes in water as a function of temperature. Further, the imidazole component of red blood cell buffer is known to interact but is eliminated in these studies by frequent washout of the coronary system. The preliminary data show a marked blunting of the time - pH decay with hypothermic and cardioplegic solutions.